MPO and anti-neutrophil cytoplasmic antibody-associated vasculitis: The association between PR3- and MPO-ANCAs and active disease in AAV suggests a pathogenic role for the autoantibodies, and such a role is supported by results from animal models (9, 10) and in vitro studies showing that PR3- and MPO-ANCAs can activate neutrophils to produce reactive oxygen species (ROS) and proteolytic enzymes (11).